1. FIELD OF THE INVENTION
This invention relates to an automatic weld line copying apparatus useful for a welding operation automatically copying a weld line.
2. DESCRIPTION OF THE PRIOR ART
A conventional sensor of differential transformer type used with an automatic copying apparatus is shown in FIG. 1. A primary coil 3 is wound around a bobbin 2, a movable core 1 is mounted movably inside the bobbin, and a secondary winding including coils 4 and 5 which are wound differentially on both sides of the primary coil 3. When a probe 1a provided on the core 1 is moved, while its end point is held in mechanical contact with a material to be detected 6, an output voltage produced in the secondary winding is proportional to the movement of the core 1 relative to the coils. This type of sensor is used in an automatic copying apparatus of contact type.
Another conventional automatic copying apparatus is of non-contact type provided with a sensor which detects the distance from the base metal by utilizing the unbalance of magnetic fluxes caused by the proximity of the metal to the end surface of a fixed core 1.
In the case of the former automatic contact type copying apparatus, the copying surface must be smooth to eliminate the effect of spatter, scale, tack welding, flaw or magnetic flux on the base metal, thus limiting the applications thereof. Further, it is so bulky that it measures 20 mm in outside diameter and 50 mm in length.
In the latter apparatus of non-contact type, the sensor detection characteristics are such that at the area of relatively large distance from the base metal surface to the sensor, the output voltage V (r.m.s; effective value) is substantially constant, as shown by curve A in FIG. 2, thus making detection impossible. Further, when the sensor temperature increases, the relative position between the bobbin and the core changes, resulting in erroneous change of the output voltage of the secondary coils 4 and 5, and also the detection is readily affected by magnetic noise due to welding current.